In a power amplifier for power amplifying a linearly modulated wave or a plurality of modulated waves, there is a need to minimize nonlinear distortion for the purpose of suppressing unnecessary waves (spurious waves) and increasing the power efficiency. As conventional systems for compensation for nonlinear distortion in amplifiers, negative feedback systems, feedforward systems, predistorter systems, etc., are known.
Negative feedback systems find not many instances of use in wireless communication apparatuses for base stations because an oscillation phenomenon can occur easily therein in the case of amplification of signals in a wide band, and because the stability of the operation is thereby reduced.
Feedforward systems are considered theoretically free from instability of operation. Therefore they are presently used in wireless communication apparatuses for many base stations. Feedforward systems, however, need to have a configuration for extracting an error component, separately amplifying the error component by a sub amplifier, and subtracting the amplified error component from the output signal from a main amplifier and, therefore, have the problem that the circuit configuration is complicated and the power efficiency is reduced due to use of the sub amplifier.
Predistorter systems require no sub amplifier and therefore attract attention as systems replacing feedforward systems. Therefore, the development and study of predistorter systems are now being advanced. Predistorter systems attract attention because of their high power efficiency. A predistorter system cancels out a distortion generated in an amplifier by previously distorting the input signal. A predistorter system (predistorter) is known in which an out-of-band unnecessary wave (spurious wave) generated from a nonlinear distortion is measured and a distortion signal is generated so as to minimize the unnecessary wave. For example, there are known predistorter systems: one in the patent application “Japanese Patent Application 2000-9661” of the invention made by the inventor of the present invention, one described by Y. Nagata in the thesis “Linear Amplification Technique for Digital Mobile Communications”, Proceedings of the IEEE Vehicular Technology Conference, pp. 159-164, 1989, and one described by F. Antonio and others in “A Novel Adaptive Predistortion Technique for Power Amplifiers”, Proceedings of IEEE Vehicular Technology Conference, pp. 1505-1509, 1999.
In general, the gain of a power amplifier changes easily depending on an environmental condition, e.g., the power supply voltage, ambient temperature or passage of time, and a nonlinear distortion characteristic also changes depending on such an environmental condition. Nonlinear distortion compensating power amplifiers using conventional predistorter systems have a problem that a discrepancy occurs between a distortion signal generated by a predistorter and a nonlinear distortion characteristic of the power amplifier to impair the nonlinear distortion cancellation function.